The present invention is directed to A.C. coupled gas-discharge display devices of the multi-digit or character indicator type and more particularly to a control circuit for driving such a display related to multiplexed operation of such display devices to provide error-free operation of the display device.
It is well-known that an electroluminescent cell can be interposed between first and second electrodes and that, upon the application of a suitable electric potential between the first and second electrodes connected to the cell, the cell will become luminescent because of the ionization which occurs within the cell. This characteristic lends itself quite readily for use in a display panel. A control circuit for driving such a display is shown in U.S. Pat. No. 3,614,769 which issued Oct. 19, 1971, on the application of William E. Coleman et al. and assigned to the assignee of the present invention.
As disclosed in that patent, the application of an electric field to an electroluminescent cell causes ionization to occur within the cell. The electric field imparts energy to electrons which collide with other atoms, thus releasing other electrons. This electron multiplication process continues until breakdown occurs, at which time ignition occurs, that is, a gaseous discharge occurs within the cells, causing positive charges to be deposited on the cell walls connected to the cathode and electrons to be deposited on the cell walls connected to the anode. The charges deposited on the cell walls are trapped because of the capacitive coupling effect exerted by the cell walls. Since positive ions are attached to the cathode wall and electrons are attached to the anode wall, the wall charge will be of a polarity to that of the electric field which instigated the gas discharge. In other words, the voltage contributed by the wall charge will be opposite in polarity to the applied electric field. Thus, it can be seen that, after discharge occurs, the total voltage impressed on the cell will be the algebraic sums of the voltages applied to the cell terminals plus the voltage contributed by the wall charge, which after ignition is negative with respect to the applied voltage, therefore resulting in a decreased cell voltage. The gas discharge which occurs in the cell continues until the wall voltage builds up to a certain value. Its value is given by the relationship V.sub.a -V.sub.w &lt;V.sub.e, where V.sub.a is the applied voltage, V.sub.w is the wall voltage, and V.sub.e is the voltage below which the cell is extinguished. In order to energize the cell again using the same magnitude of applied voltage, it is necessary to reverse the polarity of the applied voltage to the cell, thereby impressing an applied voltage across the cell which is adequate with the wall voltage left from the previous discharge, thus permitting a gas discharge to occur in the reverse direction. Since the wall charge is trapped within the cell, the wall voltage will always oppose the voltage which initiated the gas discharge.
Information is visually displayed in the display device in the form of characters, the characters being formed by a group of electroluminescent cells containing an encapsulated gas. The illumination is provided by a gaseous discharge within the cell which occurs upon the application of an electric field at the cell terminals, thereby igniting the cell. Control circuits are provided for selectively energizing the electroluminescent cells, each of which is capacitively coupled between two electrodes, such as a segment electrode and a column electrode. The number of segment electrodes is determined by the number of cells per character, and the number of column electrodes is determined by the number of characters in the display device. Electrically, this takes the form of a matrix in which the columns are called segment electrodes. Each individual cell connected in a column is called a segment cell, and the segment cells in each row are connected to a character or column electrode. One end of each segment electrode and each column electrode is connected to a potential source through appropriate drive transistors. The other ends of the segment and column electrodes are each connected to ground through individual driver transistors. The energization of selected segment cells in addition to a particular column electrode determines the character to be displayed. Circuit means are provided for logically controlling the drive transistors.
In order to illuminate a selected cell for display purposes, it is necessary to alternately energize the electrodes connected to the selected cells. In a multiplexing operation of each character, the cathode electrodes in each of the characters is connected to a common pull-up driver together with a selected number of characters connected to a common column driver. Each segment in the characters is connected to a segment driver. During a multiplexing operation, the display's control logic uses both the common column and segment drivers to designate which cells are to be energized. This process occurs on a scanned, "one column at a time," basis. A blanking period is required between the selection of columns to enable the segment data to be transmitted to the control logic. It was found that due to the multiplexing operation of the existing display systems, certain of the cells were not becoming fully charged prior to their "display-on" operation because of the asynchronous timing between the signals that drive the cells and the signals which determine the cell to be energized. It is therefore the object of this invention to provide a control driver circuit which prevents this misoperation of the cells in the display.